Envplate

Installation

You can install locally:

go get -u github.com/ispringteam/envplate/cmd/envplate

Or package to docker container with docker build --tag envplate . command and the following Dockerfile:

FROM golang:alpine AS builder
RUN apk add --update git && \
    go get -u github.com/ispringteam/envplate/cmd/envplate

FROM alpine:latest
COPY --from=builder /go/bin/envplate /usr/bin/envplate

Usage

Trivial templating for configuration files using environment keys. References to such keys are declared in arbitrary config files either as:

1. ${key} or

• ${key:-default value}

Envplate (envplate) parses arbitrary configuration files (using glob patterns) and replaces all references with values from the environment or with default values (if given). These values replace the keys inline (= the files will be changed).

Failure to resolve the supplied glob pattern(s) to at least one file results in an error.

Optionally, envplate can:

• backup (-b flag): create backups of the files it changes, appending a .bak extension to backup copies
• dry-run (-d flag): output to stdout instead of replacing values inline
• strict (-s flag): refuse to fallback to default values
• verbose (-v flag): be verbose about it's operations

envplate can also exec() another command by passing

• -- after all arguments to envplate
• the path to the binary and it's arguments

Example: envplate -v *.conf -- /usr/sbin/nginx -c /etc/nginx.conf

This can be used to use envplate to parse configs and execute the container process using Dockers CMD

Escaping

In case the file you want to modify already uses the pattern envplate is searching for ( e.g. for reading environment variables ) you can escape the sequence by adding a leading backslash \. It's also possible to escape a leading backslash by adding an additional backslash. Basically a sequence with an even number of leading backslashes will be parsed, is the number of leading backslashes odd the sequence will be escaped.

See https://github.com/kreuzwerker/envplate#full-example

Why?

For apps running Docker which rely (fully or partially) on configuration files instead of being purely configured through environment variables.

You can directly download envplate binaries into your Dockerfile using Github releases like this:

RUN curl -sLo /usr/local/bin/ep https://github.com/kreuzwerker/envplate/releases/download/v0.0.8/ep-linux && chmod +x /usr/local/bin/ep

...

CMD [ "/usr/local/bin/ep", "-v", "/etc/nginx/nginx.conf", "--", "/usr/sbin/nginx", "-c", "/etc/nginx/nginx.conf" ]

Full example

$ cat /etc/foo.conf
Database=${FOO_DATABASE}
DatabaseSlave=${BAR_DATABASE:-db2.example.com}
Mode=fancy
Escaped1=\${FOO_DATABASE}
NotEscaped1=\\${FOO_DATABASE}
Escaped2=\\\${BAR_DATABASE:-db2.example.com}
NotEscaped2=\\\\${BAR_DATABASE:-db2.example.com}

$ export FOO_DATABASE=db.example.com

$ ep /etc/f*.conf

$ cat /etc/foo.conf
Database=db.example.com
DatabaseSlave=db2.example.com
Mode=fancy
Escaped1=${FOO_DATABASE}
NotEscaped1=\db.example.com
Escaped2=\${BAR_DATABASE:-db2.example.com}
NotEscaped2=\\db2.example.com